Ferdinand travels to Malfi to investigate and overhears the Duchess and Antonio talking. As Antonio leaves, he confronts her. She attempts to flee into exile, but she is captured while Antonio is able to escape with their eldest child.

The Duchess is taken back to her palace at Malfi where Ferdinand's hideouspsychologicaltortures await her. He locks her in a dark room, shows her what she thinks is Antonio's and her childrens' corpses (but is actually a wax replica), and surrounds her with the insane. However, the Duchess keeps her head and dies nobly- which is more than can be said for her servant Cariola.

Antonio, not knowing any of this is happening, attempts a reconciliation with Ferdinand and the Cardinal. Meanwhile, Bosola, moved by the Duchess' death (and Ferdinand's refusal to pay him) seeks to kill the brothers. Spying on the Cardinal, he manages to obtain the keys to their house and sneaks in. Unfortunately, he mistakes Antonio for the Cardinal and kills him instead. The Cardinal and Ferdinand arrive, and the three manage to kill one another, just in time for Antonio's eldest child to arrive and take control of the country which is rightfully his.

Finally, it is worth mentioning Webster's ever-shifting viewpoint. During the play he never allows the audience to make up their mind about an issue, whether it is Antonio's marriage above his station or Bosola's morality. Each character justifies his actions, although some, of course, are more easily justified than others.